Method and device for electrically controlling the warp tension in looms for weaving



1963 H. LOCHER 3,072,154

METHOD AND DEVICE FOR ELECTRICALLY CONTROLLING THE WARP TENSION IN LOOMS FOR WEAVING Filed 001;. 22. 1959 5 Sheets-Sheet 1 Jan. 8, 1963 H. LOCHER 3, METHOD AND DEVICE FOR ELEGTRICALLY CONTROLLING THE WARP TENSION IN LOOMS FOR WEAVING Filed Oct. 22. 1959 5 Sheets-Sheet 2 Jan. 8, 1963 H. LOCHER 3,072,154

METHOD AND DEVICE FOR ELECTRICALLY CONTROLLING THE WARP TENSION IN LOOMS FOR WEAVING Filed Oct. 22. 1959 5 Sheets-Sheet 3 Fig.6

Jan. 8, 1963 H. LOCHER 3,072,154

METHOD AND DEVICE FOR ELECTRICALLY CONTROLLING THE WARP TENSION IN LOOMS FOR WEAVING Filed Oct. 22. 1959 5 Sheets-Sheet 4 Jan. 8, 1963 H. LOCHER 3,072,154

METHOD AND DEVICE FOR ELECTRICALLY CONTROLLING THE WARP TENSION IN LOOMS FOR WEAVING V Flled Oct 22, 1959 5 Sheets-Sheet 5 limited States Patent METHOD AND DEVICE FOR ELECTRHIALLY CONTROLLING THE WARP TENSZUN 1N LfiOMS FOR WEAVING Hans Locher, Uster, Switzerland, assiguor to Zellweger Ltd, Uster Factories for Apparatus and Machines, Uster, Switzerland, a corporation of Switzerland Filed st. 22, 1959, Ser. No. 847,988 Claims priority, application Switzerland Get. 22, 1953 11 Ciaims. (Cl. 139-410) The present invention relates to a method and device for electrically controlling the warp tension in looms for weaving.

Warp let off motions forming part of looms for Weaving serve the following purposes:

(a) Keeping the warp tension constant when weaving oh? the warp from the full beam to the empty beam;

([2) Letting off the same amount of warp at every pick;

(c) Compensation of the warp tension variations which are caused by the movement of the shafts effecting the shedding.

Keeping the warp tension constant is of extreme importance for the Weaving process itself, as well as for the uniformity of the appearance of the finished fabric. If, during weaving, high static and in particular high dynamic warp tensions occur, the danger of thread breakages is considerably increased, whereby the efficiency of the loom is reduced due to frequent loom stoppages.

A disadvantage of many of the known let off motions is that they deliver only the Warp required for each weft insertion, butdo not provide the extra length of warp material needed for each Opening of the shed, most of which is returned when closing the shed. The conven tional devices depend, for this purpose, on the elasticity of the warp threads, so that the warp threads are periodically additionally tensioned.

Resiliently supported whip rolls have been proposed. They have the disadvantage that, due to the oscillating masses, the compensation occurs too late so that, under certain circumstances, instead of the desired compensation, dynamic tensions are produced, increasing the warp tension. For instance, looms are known which supply the extra length of warp, periodically required by the shedding operation, by a drive of the whip roll, oscillating the latter synchronously with the shedding operation, whereby most of the otherwise periodically occurring additional tension of the warp threads is avoided.

Such drives of the whip roll, however, require additional mechanisms and, in most cases, are unable to guarantee a full compensation during weaving off of the entire warp.

To provide a uniform warp let off, various methods and devices are in use. According to the type of loom and to the material to be woven, positive or negative let off motions are used. The negative let off motions, which are based on the principle of friction, have the disadvantage that the warp beam is forcibly oscillated by the warp whereby the temporary dynamic warp tension variations are considerably increased. In the case of such negative let off, a brake arrangement acts on the warp beam which is released upon an increase of the warp tension beyond a predetermined tension, whereby the warp beam may be turned to such an extent that the warp tension drops for the period of one or more weaving cycles.

In the case of the positive let off motion, the warp beam is driven mechanically so that the warp required for the weaving operation is automatically supplied. For this purpose, for instance, stepwisely acting drives are provided, turning the warp beam through a certain angle after each pick. The positive let off motion does not have the disadvantage of the negative let off motion. However,

Fine

in this case, the decrease in diameter of the wound warp must be compensated by complicated arrangements.

Let off motions which maintain the warp tension by means of mechanical arrangements have the disadvantage that, after a loom stoppage, or a manipulation on the warp during which the warp tension has been arbitrarily changed, several picks, under certain circumstances ten or even more, are required until a desired stable average warp tension is effected by means of the mechanical warp tension control. As a result, the appearance of the piece of cloth woven during this period is definitely different from that of the rest of the cloth, due to the different spacing of the weft threads. These irregularities occur after each loom stoppage and impair the quality of the cloth. Certain conventional systems of let off motion form a closed control loop in which a mechanical magnitude is measured and a signal produced thereby is mechanically controlling this mechanical magnitude. Since such mechanical systems do not include amplifiers, the control either requires too much time or is inaccurate.

It is an object of the present invention to provide a method and apparatus for overcoming the disadvantages of conventional systems for maintaining the warp tension when Weaving and includes an automatic warp let off, satisfying the warp requirements of each weft insertion as well as of each change of the shed, whereby the warp tension is measured somewhere between the cloth beam and the warp beam without appreciable delay by means of a measuring device for obtaining a first electric signal U corresponding to the warp tension P; this signal is compared with a second electric signal U corresponding to the set point of the warp tension for producing a third electric signal U corresponding to the difference between the first and second electric signals and which third electric signal U is fed to the input terminals of an electric amplifier which amplifies the third electric signal U the amplified signal U actuates an electric-mechanical motor operator which has a driving shaft, the direction of rotation of which changes when the polarity of the unamplified signal U is changed, and the speed of rotation of which shaft corresponds to the amplitude of the signal U the driven shaft is mechanically connected to the warp beam whereby. in the case of increasing warp tension P, the warp beam is turned in the direction of the warp let off and, in the case of decreasing warp tension, in the direction of wind up, so that the warp tension P deviates only slightly from its set point.

A further object of the invention is the provision of an apparatus, in looms for weaving, for controlling the move ment of the warp beam to maintain a substantially uniform warp thread tension during weaving, the apparatus including a device for producing a substantially undelayed first electric signal corresponding to the warp tension P between the Warp beam and the cloth beam, a source of voltage for producing a second electric signal U corresponding to the set point of the warp tension P, means for forming a third electric signal U which corresponds to the difference between the first two electric signals U and U an electric amplifier for amplifying the third electric signal U to produce an amplified signal U and an electric-mechanical motor operator which is actuated by the amplified signal and has a driving shaft, the direction of rotation of which changes when the polarity of the electric signal U changes and whose speed corresponds to the amplitude of the electric signal U the driving shaft being mechanically connected to the warp beam.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, and additional objects and advantages thereof will best be understood from the following description of embodiments thereof when read in connection with the accompanying drawing, in which:

FIG. 1 is a diagrammatic illustration of the fundamental parts of a loom and of an apparatus according to the invention.

FIG. 2 is a diagrammatic illustration of an electric warp tension measuring device and its stroke-voltage diagram.

FIG. 3 is a diagrammatic illustration of a modified electric warp tension measuring means and its strokevoltage diagram.

FIG. 4 schematically shows another modification of an electric warp tension measuring device.

FIGS. 5 and 6 are diagrams showing the relation between input signal and output signal of two modifications of an electric amplifier forming part of the system according to the invention.

FIG. 7 is a schematic illustration of an electric-mechanical motor operator.

FIG. 8 is a diagram showing the movement of a shaft and of the simultaneous warp tension in a loom during a weaving cycle.

Referring more particularly to FIG. 1 of the drawing, numeral 27 designates a diagrammatically shown loom frame supporting a warp beam 1 from which warp threads 2 are undwound. The warp formed by the threads 2 passes over a whip roll 5 into the eyes of heddles 18. The latter form a shed having a rear portion 19 and a front portion 19'. Weft threads, not shown, are picked through the front portion 19 for producing a fabric. The latter passes over a breast beam 23, around take up rolls Z4, and therefrom to a cloth beam 26. The loom is driven by a main driving motor 2% driving a crank shaft 2%, for example, by means of a belt. The crank shaft reciprocates through connecting rods 21 a lay 22. The crank shaft 26 drives, by means of gears, not shown, a cam shaft 14 for actuating the heddles or shafts 18. The loom includes conventional shuttles and means for picking the shuttles which are not shown, because they do not form part of the present invention.

The method according to the invention makes use of a control loop in which the tension P of the warp threads is measured at a suitable point between the warp beam 1 and the cloth beam 26, and is transformed in a device 9 into an electric signal. The latter is amplified in an amplifier 4t and used for actuating an electric-mechanical control means ltl. The latter controls, for example, the rotation of the warp beam 1, whereby the warp 2 is more or less tensioned so that the warp tension P is automatically maintained to correspond to the set point of the system. Theoretically, the same result could be obtained by suitably moving the whip roll 5. By suitable amplification of the electric signal produced in the device the accuracy and speed of the control operations can be made so great that the warp thread tension P is maintained within very narrow limits.

Since it is desired to place the heavy warp beam 1 as low as possible, a whip roll 5 is necessary in order to support the Warp 2 approximately at the elevation of the shafts 18. The whip roll 5 must, therefore, absorb the entire force which is produced by the change of direction of the warp. A spring 8 may be provided against which the whip roll is pressed by the horizontal component P of the total warp tension P acting on the Whip roll.

counteraction of the horizontally acting warp tension P which is a component of the force 1 acting on the whip roll 5 is preferred, because only the warp tension 'P is of importance at the locality of the weft thread insertion and of the beating-up operation. The entire force P acting on the whip roll 5 does not exactly correspond to the horizontally acting Warp tension P because of the friction between the warp and the whip roll.

Changes of the warp tension 9 change the pressure of the spring 8 and change the position of the whip roll 5 relative to the frame 27. The Whip roll 5 is supported by means of arms 6 in bearings 7. A spring 8 is usually 4 provided at each arm 6 so that the position of the whip roll is different at different warp tensions and depends on the force-stroke characteristic of the springs.

The position of the whip roll 5 is transferred to the armature 41 of an electric induction device including a stator and a coil 42. An alternating current J- is conducted through the coil. Displacement of the armature 41 to the extent S (FIG. 2) causes a change of the space between the armature and the stator and thereby a change of the inductance. Thereby the amplitude of the alternating current passing through the coil 42 and the voltage thereat are changed. The armature 41 is pressed by a spring 39 (PEG. 2) against an arm 6 supporting the whip roll 5. The alternating voltage occurring at the coil 42 is conducted to a rectifier 44 including filtering means 43 for rectifying the alternating voltage and producing a first electric signal U corresponding to the warp tension P.

If the relation between the first electric signal U and the position of the whip roll 5 as well as the characteristic of the spring 8 are known, the signal U can be used to directly indicate force units, for example pounds, of the warp tension P. A whip roll which is movably supported against springs is particularly suitable for this purpose. Without departing from the scope of the invention, the warp tension may be measured by suitable means at any other location between the warp beam 1 and the cloth beam 26. The set point, i.e., the desired tension P of the warp is preferably also represented by an electric magnitude. A source of electric voltage 34 and a potentiometer 38 supply a second electric signal U corresponding to the set point of the warp tension P acting on the whip roll 5. This set point can be changed within predetermined limits by means of the potentiometer 38. The negative pole of the source 34 is connected, for example, to the negative pole of the rectifier 44. In this way, a third electric signal U can be obtained between the positive pole of the rectifier 44 and the tap of the potentiometer 38 as the difference between the first electric signal U and the second electric signal U This third signal may be positive, zero, or negative, depending on the magnitude of the first signal relative to the second signal. In the diagram forming part of FIG. 2, the first electric signal U is shown as a function of the movement of the whip roll 5 to the extent +5 or S. The diagram also shows the second electric signal U which; forms a line parallel to the abscissa, the distance from the abscissa corresponding to the position of the sliding contact of the potentiometer 38. The difference between the two signals U and U is represented by a line U parallel to the line representing the signal U A change of the second electric signal U by means of the potentiometer 38 by the value AU changes the distance between the line representing U and the line representing U by the value AU so that the curve U +AU intersects the abscissa at a distance AS from the O ordinate. The distance AS corresponds to a change of position of the whip roll 5 which corresponds to the new set point of the warp tension P caused by the change of the-second electric signal U -l-AU FIG. 3 illustrates a modification of a measuring device 9 which has similar characteristics as the device shown in FIG. 2. The apparatus shown in FIG. 3 includes a differential transformer having a stator 42' and a movable armature 41'. The stator 42' comprises two primary coils arranged in series relation through which passes an alternating current and two galvanically separated secondary coils. Each of the latter is connected to a rectifier 44, 47. As long as the movable armature 41 is in the middle between the two secondary coils, the voltages induced in these coils are equal. After rectificatjon in the rectifiers 44' and 47, the first electric signal U and the second electric signal U are equal and the third electric signal U amounts to zero. If" the whip roll and, consequently, the movable armat ltq 4 1" are;

7 signal U and producing an moved from the middle position, for example to the right, the relative size of the air gaps within the stator 42 is changed so that the first electric signal U for example, increases and the second electric signal U becomes smaller. Therefore, the third electric signal U will have a positive value. If the armature 41 moves to the left, the third electric signal U becomes negative.

The mutual dependency of the first electric signal U and the second electric signal U necessitates adjustability of the set point of the warp tension P by displacement of the stator 42 in the frame 27 of the loom in the direction of the movement of the armature 41 by relatively simple means, for instance, by an adjusting screw. This displacement is preferably so effected that, at a certain characteristic of the spring 8, the stator 42' is in certain positions at certain average warp tensions P.

FIG. 4 illustrates a measuring device 9" stator 78 and a rotor 79 of an electric rotating plate condenser. The rotor 79 is operatively connected to and moved by the whip roll 5 relative to the stator 78. In principle, the transformation to electrical magnitudes AU of the capacity changes AC caused by the movement of the rotor 79 can be effected in the same manner as in the inductively actuated measuring means 9 according to FIG. 2.

The third electric signal U produced in the measuring device 9', shown in FIG. 3, by the rectifiers 44, 47 with attached filtering means 48, 49, respectively, can be made visible by a suitable indicating instrument 37 (FIG. 1) and can, if desired, be recorded.

Since the third electric signal U corresponds to the deviations of the warp tension P from its set point and since these deviations are defined by the known characteristics of the springs 8, the scale of the instrument 37 can be so designed that the Warp tension P is directly shown, for instance, in pounds or kilograms.

The third electric signal U passes through a system 30 including a potentiometer 31, a condenser 32 and a resistor 33. By means of the potentiometer 31, the magnitude of the input signal U received by a subsequent electric amplifier 4! can be adjusted. The condenser 32 has a plurality of steps by means of which the correct phase of the input signal U which is obtained from the third electric signal U can be produced for the amplifier 46 to satisfy the operating conditions of the loom and of the warp tension control device according to the invention. Adjustment of the phase of the input signal U is necessary because there is a small difference of the time between the movement of the harnesses and the weft insertion and the effect thereof on the warp requirement. These differences of time can be compensated by the phase adjustment by the condenser 32 so that optimum efiicency of the warp tension control is obtained.

FIGS. 5 and 6 show the relation between the input signal U and the amplified signal U of two differently designed electric amplifiers 49. In FIG. 5, the amplified signal U is directly proportional to the input signal U For positive values of U the amplified signal U, will also be positive (U for negative values of U the amplified signal will also be negative (11 FIG. 6 relates to an electric amplifier producing an amplified signal U at a first output terminal from a positive input amplified signal U," at a second output terminal when a negative input signal U appears, the amplified signals U and U being very small and equal, if the input signal U =0.

The amplified signals U and U are conducted over separate lines to a motor operator (FIG. 1), an example of which is schematically shown in FIG. 7. It is supplied with mechanical power produced either by the main driving motor 29 of the loom, or by a separate constant speed motor. The motor operator includes a drive shaft 67 rotating the warp beam 1 through a Worm including a 6 gear 3, 4 in one or in the opposite direction of rotation to either release or wind the warp.

The control of the motor operator 1& according to the electric signals U U is eifected by means of electrically controlled brakes 11, 12. A pinion 61 connected to a pulley 13, driven by the motor 29, drives, at a substantially constant speed 11;, a gear 62 connected to a cage 63 of a first planetary gear 60. The cake 63 supports a planetary gear 64. The driven shaft 67 carrying a sun Wheel 65 extends through the gear 62 to the outside and drives the Worm gear 4 through speed reducing bevel gears 71, 72. A second driven shaft 68, carrying a sun Wheel 65, extends through the right side of the cage 63 and supports a pinion 65.

A second differential gear 59 includes a gear 52 which is driven at the substantially constant speed 21 by the gear 62, or by a separate motor. A cage 53 supporting a planetary gear 54 is connected to the gear 52. A first driven shaft 57 carrying an sun wheel 56 extends through the gear 52 to the left in FIG. 7. The left end of the shaft 57 forms part of a first electrically controlled brake 11. A second electrically controlled brake 12 is connected to the right end of a second driven shaft 58, to the left end of which a sun wheel 55 engaging the planetary gear 54 is connected.

A gear 59 is fast on the shaft 5'8 and engages the pinion c9 of the first differential gear 6%) to form an intermediate gearing 7d.

The rotors l1 and 12 of the electrically controlled brakes It}, 32, which rotors are connected to the free ends of the shafts 57, 58, respectively, can be slowed down continuously to a standstill by connecting a suitable voltage U ,U to the terminals of the stator coils of the brakes. With the illustrated combination of the diiferential gears 55% and 6t and of the electrically controlled brakes 11 and 2., the speeds and directions of rotation listed in the following table are obtained, provided that the speeds n and 11 of the oppositely rotating gears 52, 62. are equal, and that the speed 11 (shaft 68) is twice as great as the speed in or" the shaft 53 the shafts 68 and 58 rotating in opposite direction.

Brake 11 Brakes 11 Brake 12 inactive, and 12 ininactive, Brake 12 active Brake 11 active active Gear 62 m n1 1 Gear 52 n -12; -m 'm m 2n -2n1 71g=7l1 0 riven shaft 58 (rotor 12) m: 0 n =-n1 2m=-2n Driven shalt 6S n 0 -27tz=+2tt1 41rz=+4m Driven shaft 67 n +2m 0 --2m By only partly exciting the electrically controlled brakes 11, 12, the driven shaft 67 of the first differential gea r 60 may be rotated at any speed between +211 and 2ll v. A ratio 11 112 of the gears 62 and 52, different from -1. may be employed, provided that the ratio 11. 121 of the intermediate gearing 76 is changed likewise. This affords high speed operation of the rotors 11, 12 of the electrically controlled brakes ll, 12 so that the shafts 57, 58, because of the small torque, can be slowed down to a standstill by weak signals U The electric-mechanical motor operator 14] according to the invention has several advantages over other electric-mechanical motor operators. The electrically controlled brakes 11 and 12 may be exclusively electrically actuated or may be of the electric-magnetic type, of the electric-mechanical type, or of the electric-hydraulic type. Of particular advantage are magnetic powder brakes. The latter have very small rotating masses so that the brakes H and 12 can be operated at high speeds H and n and only little electric power is needed for producing the desired brake moment, which power can be obtained by an electric amplifier 40 which can be of simple design. As a consequence of the small rotating masses, natural frequency of the entire electric-mechanical motor operator is high so that the motor operator can produce the necessary, very quick variations of the movement of the warp beam without undue delay.

The motor operator may also be of a type in which the signal U amplified by the amplifier 40, is directly transformed into mechanical rotation. Such a motor operator is in the form of a motor through which the amplified signal U is conducted. In this case, the amplifier 40 must have a characteristic as illustrated in FIG. 5, wherein magnitude and polarity of the amplified signal U is proportional to the input signal U in this case, preferably a DC. motor is used which, however, requires considerable power which must be supplied by the electric amplifier 40. The rotor of such a motor has a relatively great moment of inertia so that the reversal of the direction of rotation of the warp beam 1 can hardly be effected within the short periods of time available therefor.

In most looms for weaving, it is of advantage if the warp supply mechanism does not only deliver the warp necessary at every pick, but also additional warp temporarily required for the shedding operation. This additional warp supply is needed at every change of shed and requires warp let off and warp take up at every weaving cycle. The extent and timing of this additional warp requirement depends on the characteristic of the movements of the shafts 18 The top part of FIG. 8 shows the shaft stroke H during one weaving cycle below which the warp tension P is plotted, if there is no or an insufficient warp let off. At the moment A, the shafts 18 are in closed shed position and at the moment B in open shed position in which they remain up to the moment C when a change of shed begins. The shed is closed at the moment D, whereupon it opens and is fully open between the moments E and F. The warp tension P correspondingly changes from a minimal value at the moment A to a maximum value between the moments B and C, drops during the change of shed to a minimum tension at the moment D, whereupon it rises again to the maximum value which lasts from the moment E to the moment F. The time periods A--B, B-'C, C-D, D-E, E-F, F-G correspond to the characteristic of the shedding mechanism. The subdivision of a shedding cycle into three times 120 is arbitrary, but approximates the conditions prevailing in most shedding mechanisms.

The varying warp tension P actuates the measuring device 9 and causes an oscillating movement of the warp beam 1 and let off and take up of the warp. It is obvious that a control of the additionally required warp based exclusively onthe warp tension P cannot be correctly timed because, due to the inertia of the parts to be moved, the warp let off does not begin to act until the additional warp demand has produced a considerable increase of the warp tension. Likewise, a warp take up is not effected until the warp tension has been reduced to a value which is much below the permissible value.

In order to initiate the additional supply of warp required by the shedding operation at the correct moment, the invention provides means which synchronize the acceleration of the warp beam 1 accurately to the movement of the shafts so that the warp let off has already begun when the shed 19 is opened, and the warp take up has already begun when the shed 1? is closed. These means include two switches 16 and 17 (FIG. 1) which are actuated by a cam which is connected to an element controlling the movement of the shafts 18. This element may be the cam shaft 14 of the loom. The switches l6 and 17 are interposed in a circuit including a source of electricity 46 and a potentiometer 45. Closing of the -C, D, etc.

switches 16 and 17 produces signals U and U which are fed into the amplifier 40. The amplitude of the signals U and U is, at all operating conditions, greater than the third electric signal U and the input signal U of the amplifier 40. The duration of and the moments when the signals U and U occur are so that the motor operator it? moves the warp beam l in one or in the opposite direction of rotation to an extent that the warp let off or the warp take up at the moments A and C, respectively, (FIG. 8) is always initiated at the proper time and with the correct amplitude. The subsequent control of the warp beam 1 between the fading out of the previous signal and the arrival of the subsequent signal is effected by the third electric signal U which corresponds to the warp tension P. The signals U and U are fed into the amplifier 40 at an angle of lead go ahead of the moments A and C, so that the speeds of rotation of the masses taking part in the rotation of the warp beam have already attained the required values at the moments A, Since the interval between the moments A and C amounts to 240 and the interval between the moments C and D amounts to the switches 16 and 17 must be placed at the same angles relative to the cam 15 so that the switches 16 and 1'7 are consecutively actuated by the cam 15. In cases in which the movement of the shafts per weaving cycle is not divided into three equal periods corresponding to an angular movement of the cam shaft of 120 per period, the switches 1e and 17 must be placed in different angular positions. Since the cam shaft 14 rotates at one half of the speed of the crank shaft 20 of the loom, two earns 15 are provided which are spaced 180 and the switches 16 and 17 are spaced 60 (or). The angle of lead (p may be made adjustable by providing for adjustment of the angular position of the cam 15 on the cam shaft 14. The angle of lead g0 must be experimentally determined for each loom.

The operation of a loom requires that in case of a standstill, the warp tension P is reduced to a lower value than is needed for weaving. For this purpose, a switch 35 (FIG. 3) is interposed in the electric circuit for the second electric signal U the switch 35 being connected to the loom stop motion. A potentiometer 38 affords adjustment of the reduction of the warp tension P. The second electric signal U and, consequently, the set point of the warp tension P is reduced upon closing of the switch 35 so that also the first electric signal U will be reduced because of the resulting reduction of the warp tension I. Upon restarting of the loom, the switch 35 is opened so that the initial values of the second electric signal U and of the set point of the warp tension P are restored. This causes rotation of the warp beam 1 until the first electric signal U has attained the initial value. When restarting the loom, the warp take up must be effected so quickly that the correct warp tension P is restored at the first throw of the reed by the lay 22, at the latest.

A further requirement of loom operation is the possibility of letting off so much warp 2 that not only the warp tension P is reduced to zero, but that the warp 2 sags. To accomplish this, the invention provides a step switch 36 which can be manipulated by hand. The step switch includes a part 36 (FIG. 3) in the circuit of the signal U and a part 36" (FIG. 1) in the circuit of the amplified signal U and can be placed in three different positions (FIGS. 1, 3). In a first of the three positions, the switch part 36 is opened and the switch part 36 is closed so that the switch 36 has no effect on the electrically controlled warp let ofi motion. in a second position of the step switch 36, the part 36' is closed so a first electric signal U is produced which cannot be compensated by any value of the second electric signal U Therefore, the third electric signal U entering the electric amplifier 40 is different from zero and the motor operator 10 receives an amplified signal U as long as the switch 36 is in the second operating position. When enough warp is let off, the step switch 36 may be moved to a third position in which the part 36 is open. The amplified signal U is thereby disconnected from the motor operator 10 so that warp let off is stopped, and a desireed sag of the warp 2 is maintained as long as the step switch 36 is in the third operating position. Prior to continuing weaving, the step switch 36 must be placed into the first operating position so that the correct warp tension P is restored by re-winding the released warp 2 on the warp beam 1. The step switch 36 is preferably so constructed that its first operating position corresponds to its rest position, whereas the step switch must be held by hand so long in the second operating position as it is desired to let off warp. Provision of optical indication connected to the step switch 36 is desirable to show whether the warp tension is suitable for weaving.

It is within the scope of the present invention, to control the warp let 01f solely to satisfy the warp requirement at every pick and to supply the additional warp needed for the shedding operation by an oscillating movement of the whip roll 5 which is controlled by the signals U and U which are initiated by the switches 16 and 17.

What is claimed is:

1. A method for maintaining the tension of the Warp supplied from a warp beam in a loom for weaving, including producing a first electric signal corresponding to the warp tension, comparing said first signal with a second electric signal corresponding to the set point of the warp tension for producing a third electric signal corresponding to the difference between the first two signals and having a certain polarity when the warp tension exceeds the set point and having the opposite polarity when the warp tension is below the set point, amplifying said third signal for producing a fourth signal, actuating an electricmechanical motor operator including a shaft by means of said fourth signal for changing the rotation of the shaft upon a change of the polarity of said fourth signal and for rotating said shaft at a speed corresponding to the amplitude of the fourth signal, and rotating the warp beam at a direction and speed corresponding to the direction and speed of rotation of said shaft whereby the warp beam releases warp at increasing warp tension and winds up warp at decreasing warp tension.

2. In a loom for weaving comprising a warp beam for letting off and taking up warp threads, shafts for forming a shed of the warp threads supplied by the warp beam, and drive means for driving said warp beam and said shafts: means for measuring the tension of the warp formed by the warp threads after the warp has left said Warp beam, means connected to said measuring means for producing a first electric signal corresponding to the tension of the warp, a source of voltage producing a sec- 0nd electric signal corresponding to the set point of the warp tension, means connected to said means for producing a first electric signal and to said means for producing a second electric signal for producing a third electric signal corresponding to the difference between the first two signals and having a certain polarity when the warp tension exceeds the set point and having the opposite polarity when the warp tension is below the set point, an electric amplifier connected to said means producing said third signal for receiving and amplifying the third electric signal for producing a fourth electric signal, an electric-mechanical motor operator including a rotatable shaft and being connected to said amplifier for receiving said fourth electric signal to be actuated by said fourth signal for reversing th direction of rotation of said shaft upon a change of polarity of said fourth signal and for controlling the rotational speed of said shaft to correspond to the amplitude of said fourth signal, said shaft being operatively connected to said warp beam for rotating the latter at a direction and speed corresponding to the direction and speed of rotation of said shaft.

3. In a loom for weaving as defined in claim 2 and wherein said motor operator includes a first differential gear having a cage, means connected to said cage for driving said cage at substantially constant speed, said first differential gear having a first driven shaft which is said rotatable shaft of said motor operator, said first differential gear having a second driven shaft, a second differential gear having a cage, means connected to said last mentioned cage for driving same at substantially constant speed, said second differential gear having a first driven shaft, a first electrically actuated brake connected to said first driven shaft of said second differential gear, the latter having a second driven shaft, and a second electrically actuated brake connected to said last mentioned second driven shaft, said second driven shaft of said first differential gear and said second driven shaft of said second difierential gear being operatively interconnected, actuation of one of said brakes causing rotation of said rotatable shaft in one direction and actuation of the other of said brakes causing rotation of said rotatable shaft in the opposite direction, said brakes being electrically connected to said means producing said fourth electric signal for actuating one of said brakes at a certain polarity of said third signal and for actuating the other of said brakes at the opposite polarity of said third signal.

4. In a loom for weaving as defined in claim 3, and having a motor for driving the loom, said motor being connected to the cage of said first differential gear for driving said cage.

5. In a loom for weaving according to claim 4 and wherein said cage of said second differential gear is operatively connected to said cage of said first differential gear to be driven thereby.

6. in a loom for weaving as defined in claim 2 and wherein said means for measuring the warp tension includes a stationary element, a member movable relative to said stationary element and resting against the warp, and a spring interposed between said element and said member for counteracting movement of said member relative to said element, whereby the relative movment of said element and of said member corresponds to the warp tension, said means producing said first electric signal being connected to said member and responsive to the position of the latter relative to said element.

7. In a loom for weaving according to claim 6, and wherein said means for producing said first electric signal includes an electric circuit, a source of alternating current connected to said circuit for feeding electric current thereinto, a variable inductor having a stator including a coil interposed in said circuit, an armature movable relative to said stator and connected to said member for movement relative to said stator according to the warp tension, and a resistor interposed in said circuit in series relation with said coil, whereby the voltage at said coil corresponds to the warp tension and forms said first signal.

8. In a loom for weaving according to claim 6, and wherein said means for producing said first electric signal includes a variable capacitor having a stator and a rotor, the latter being movable relative to said stator and connected to said member for movement according to the warp tension, said stator and rotor forming a condenser whose capacity corresponds to the relative position of said stator and of said rotor, an electric circuit, a source of alternating current connected to said circuit for feeding alternating current thereinto, said stator and rotor and a resistor being interposed in series relation in said circuit, whereby the voltage at said capacitor corresponds to the warp tension and forms said first signal.

9. In a loom for weaving as defined in claim 2, a combination of a condenser and a potentiometer forming a phase shifter interposed between said means for producing -a third signal and said amplifier, for timing said third signal according to the pattern or" the warp tension changes.

10. In a loom for weaving as defined in claim 2, switch means connected to said means for producing the third electric signal for stopping the effect of said. second electric signal, and switch means interposed between said means producing said fourth electric signal and said motor operator for stopping said motor operator.

11. In a loom for weaving as defined in claim 2, indicating means connected to said means for producing the third electric signal for indicating the deviation of the warp tension from the set point of the warp tension.

References Cited in the file of this patent UNITED STATES PATENTS Kovalsky Feb. 25, 1936 Jacques Nov. 11, 1947 Lewis et al July 22, 1958 Poschner et al Mar. 10, 1959 

